Flow measurements are often used in a drilling system when drilling a borehole. For example, in a managed pressure drilling (MPD) system, pump stroke counters attached to mud pumps can be used to estimate the high pressure flowrate into the borehole (inflow). Additionally, a flowmeter can be used to make measurements of the flowrate of drilling fluid out of the borehole (outflow) as the drilling fluid returns at high pressure from the wellbore to surface equipment.
To help control pressure in the MPD system, the flow rate measurements for the inflow and outflow are preferably accurate across a full range of flow in the system's operation, from almost no flow to a maximum flow. Using stroke counters to measure inflow can be mechanically problematic because they do not measure the density of the drilling fluid nor take into account inefficiencies of the mud pump. In the system, density of the drilling fluid for the inflow can be measured at atmospheric pressures and conditions, but these measurements cannot be done on a continuous/real time basis beneficial to automated control. Besides, such density measurements may lack accuracy because the properties of the drilling fluid, as a thixotropic fluid with entrained gas, change when the drilling fluid is dynamically flowing and under pressure. In addition to the inability to measure density, the stroke counter can produce false flow rate readings when the pump cylinders are running but no flow is produced due to plugging or extreme downstream pressure.
To measure the outflow of the drilling fluid from the borehole in the MPD system, a Coriolis flowmeter can measure both density and flow rate, but the Coriolis flowmeter has pressure limitations for sizes large enough for drilling applications. For this reason, the Coriolis flowmeter is typically installed after a choke manifold in the MPD system to measure flow out of the borehole (outflow). The drilling returns from the wellbore must therefore flow through the choke(s) of the manifold to lower the flow's pressure before the Coriolis flowmeter can measure the fluid flow and/or mass flow. This is due to the pressure limitations of the Coriolis flowmeter. Also, during the pressure drop through the choke, cavitation and/or gas flashing may occur that can prevent the Coriolis flowmeter from obtaining an accurate flow rate.
Apart from measuring flow rate using pump stroke counters and Coriolis flowmeters, it is known in the art to use an orifice plate, V-cone, or other device to create a pressure differential in a flow path so the pressure differential can be measured with a differential pressure transducer (DPT). This measured difference in pressure can then be correlated to a flow rate through a flowline.
As a brief example, FIG. 1A shows a flowmeter 50 of the prior art having a differential pressure transducer 70 on a flowline 60 that uses an orifice plate 62 to measure flow rate. As shown, the orifice plate 62 reduces the pressure in the flow so that an upstream pressure P1 is greater than a downstream pressure P2. The pressure transducer 70 connects by tubing 72 to an upstream inlet 74 subject to the upstream pressure P1 and connects by the tubing 72 to a downstream outlet 76 subject to the downstream pressure P2.
As another brief example, FIG. 1B shows a flowmeter 50 of the prior art having a differential pressure transducer 70 on a flowline 60 that uses an V-cone 64 to measure flow rate. As shown, the V-cone 64 reduces the pressure in the flow so that an upstream pressure P1 is greater than a downstream pressure P2. The pressure transducer 70 connects by tubing 72 to an upstream inlet 74 subject to the upstream pressure P1 and connects by the tubing 72 to a downstream outlet 76 subject to the downstream pressure P2.
During operation, the transducer 70 in either flowmeter 50 of FIGS. 1A-1B measures the pressure difference given by ΔP=P1−P2. The flow rate of the main flow (MF) through the flowline 60 is proportional to this pressure differential, where Flow Rate∝ΔP.
For example, mass flow for liquids can be calculated from the measured pressure difference ΔP using an equation:m=CEAt√{square root over (2Δpρ)}where:                C is a discharge coefficient,        At is a throat area (restriction) of the pressure device,        Δp is the differential pressure,        ρ is the density of the fluid,        d is a diameter of the throat of the pressure device (For a V-cone meter, d is a diameter of the V-cone at its largest),        D is the pipe diameter,        
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If the flowmeter 50 having the pressure transducer 70 in FIGS. 1A-1B is used in a drilling application, such as in a MPD application, the transducer 70 and the tubing 72 can become plugged or blocked by solids, cuttings, or other contaminates that are typically in the drilling fluid. The plugging can alter the readings provided by the flowmeter 50 or can damage the transducer 70. For this reason, such a flowmeter 50 may not be suitable in many drilling systems and applications.
Operators performing managed pressure drilling can benefit from accurate measurements at multiple points of a closed loop drilling system. Current devices are not available that can provide the desired measurements at certain points. Accordingly, the subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.